1. Field of the Invention
This invention relates to an ink jet head to be used in an ink jet recording apparatus which performs recording by forming droplets of ink by discharging ink and attaching the droplets onto a recording medium such as paper, etc., a substrate for the head, processes for preparing thereof and an ink jet apparatus having the head.
2. Related Background Art
The ink jet recording method is a recording method which performs recording by discharging ink (recording liquid) from a discharge opening provided at the ink jet recording head and attaching the ink onto a recording medium such as paper, etc. The method has many advantages; it generates little noise, is capable of high speed recording, and yet recording can be practiced on plain paper, etc. without use of a special recording paper. Various types of recording heads have been developed.
Among them, the recording head of the type which discharges ink from a discharge opening by permitting heat energy to act on ink as disclosed in Japanese Laid-open Patent Application No. 54-59936, German Laid-open Patent Application (DOLS) No. 2843064 and U.S. Pat. No. 4,723,129 has such advantages as good response to recording signals, easy multi-formation of discharge openings, etc.
Representative constitutions of such a recording head of the type utilizing heat energy as the ink discharging energy are shown in FIGS. 1A and 1B.
The recording head has a constitution formed by bonding a substrate comprising an electrothermal transducer provided for transducing electrical energy to heat energy utilized for ink discharging arranged on the surface exhibiting insulating property of the support 1, and further, if necessary, an upper layer 4 as the protective layer provided at least on the heat-generating resistor 8 and the electrodes 3 of the electrothermal transducer to be finally positioned below the liquid path 6 and the liquid chamber 10 communicated with the ink supplying inlet 9 to a covering member 5 having a recessed portion for forming the liquid path 6 and the liquid chamber 10, etc. formed thereon.
The energy to be utilized for discharging ink in this recording head is imparted by an electrothermal transducer having a pair of electrodes 3 and a heat-generating resistor 8 positioned between the pair of electrodes. That is, when current is applied on the electrodes 3 to generate heat from the heat-generating resistor, the ink in the liquid path 6 near the heat-generating portion 8 is momentarily heated to generate bubbles thereat, and through volume change by momentary volume expansion and shrinkage by generation of the bubbles, a droplet of ink is discharged.
The upper layer as the protective layer to be provided on the heat generating resistor and electrodes of the substrate in the constitution of the recording head as described above is provided for the purpose of preventing galvanic corrosion or electrical dielectric failure at the heat generating resistor or electrodes by contact with ink or penetration of ink, and it must be free from defect and good in step coverage.
From such standpoint, various investigations have been made about the materials for constituting the upper layer and the methods for formation thereof.
For example, Japanese Laid-open Patent Application No. 60-234850 discloses a constitution using a layer formed by the bias sputtering method for the upper layer.
By forming thus the upper layer according to the bias sputtering method, defects are reduced, step coverage becomes better, and durability is improved. Also, as compared with the sputtering method, the defects can be reduced even with the same thickness, and therefore the film thickness can be made thinner. Accordingly, improvement of characteristics and cost down can be effected.
Whereas, in formation of the upper layer according to the bias sputtering method, there still remain problems to be solved. For example, in the recording head prepared by use of a substrate provided with an upper layer having a bias sputtered layer, defects such as poor printing or lowering in durability are liable to occur.
Whereas, when a layer formed by the bias sputtering method (bias sputtered layer) is used for at least one layer of the protective films made to have a multi-layer constitution in order to give functionally more characteristics as disclosed in Japanese Laid-open Patent Application No. 59-194866, there ensued the problem that a protective film of good protective function cannot necessarily be obtained.
For example, when an inorganic insulating material layer such as SiO.sub.2 is formed by the bias sputtering method, and further a high melting metal such as Ta is laminated thereon according to the sputtering method, cracks are remarkably generated particularly at the step portion (step difference portion in patterns of layer of electrodes), and peel-off of the protective layer is liable to occur at that portion, and also in the recording head prepared by use of a substrate having such constitution, breaking of electrodes and heat-generating resistors at low voltage may occur, whereby reliability is inferior and also durability in durability test such as step stress test may be low.
Thus, when a bias sputtered layer is used as the protective layer, paticularly when it is made a multi-layer constitution, not only the advantage of using the bias sputtered layer cannot be fully utilized, but also there is a problem that no protective layer of good quality can be formed.